basically flagged the image so that on next load/reload, the image
data would be converted to premul. This was very confusing to the
user, as it meant premul wouldn't take effect will the image was
reloaded, and it would also change the image data, which the user
might've been painting.
To fix this, I've removed this behaviour and instead made the premul
option apply at render time. During render while evaluating an image
texture, if the image has the premul flag set then the premul operation
is done on the texture result data, thus not touching the image data
at all.
Also, I've made premul be turned on by default.
139b612b09